Induction furnace systems for melting metal, known as a melt charge, have typically been powered by means of two types of inverter topologies, a voltage source series inverter or a current source parallel inverter. Generally, both have some advantages and disadvantages.
For a voltage source series inverter, a rectifier takes the alternating current supplied by the electric utility and converts it to direct current. This direct current is filtered and smoothed by a large capacitor bank, then applied to a full or half bridge inverter. The load circuit connected to the output terminals of the inverter typically consists of a series network formed by an induction coil and a resonant capacitor bank.
An example of a voltage source series inverter is disclosed in U.S. Pat. No. 5,165,049 to Rotman, which discloses a phase difference control circuit for an induction furnace power supply. The Rotman induction furnace controls the phase difference between the voltage and the current delivered to the load to control the power delivered to the load. The Rotman induction furnace, however, does not operate at a resonant frequency and thus does not maximize the energy transferred to the melt charge.
U.S. Pat. No. 5,343,023 to Geissler discloses an induction heater that uses pulse width modulation to control the power output to the induction heater. The Geissler apparatus, however, uses two inverters for one induction coil, one inverter that outputs at a constant frequency and another that outputs at a varying frequency to control the power output.
Advantages of the voltage source series inverter typically include simple control circuitry and full control range by means of varying the inverter firing frequency. Another advantage is the possibility to run the inverter from standard plant voltages for some ratings. Disadvantages typically include a difficulty of providing protection from arcing faults in the load coil, difficulty of clearing faults in the inverter, high resonant currents in the inverter circuitry and the difficulty of sharing current equally between many paralleled thyristors comprising the inverter structure.
The second type of inverter used in induction furnace systems is the current source parallel inverter. In this type of inverter, the three phase AC voltage supplied by the utility is converted to DC by means of a rectifier. The direct current is then passed through a large inductor which smoothes and filters the current. The filtered current is applied to a full bridge inverter. The load circuit consists of the induction coil and a parallel connected resonant capacitor bank, with a small inductor connected between the capacitor bank and the inverter output terminals for purposes of limiting the maximum rate of rise of the current.
U.S. Pat. No. 5,508,497 to Fabianowski et al. discloses the control of at least two current source parallel inverters feeding induction furnaces. For the Fabianowski apparatus, one rectifier provides an adjustable level of power for a plurality of inverters, each having its own induction furnace. The produced rectifier output at each moment is equivalent to the sum of the power of all parallel circuit inverters. Thus, only one rectifier is needed for a plurality of induction furnaces. The Fabianowski apparatus, however, operates at a frequency above the resonant frequencies of the load circuits.
Advantages of the current source parallel inverter typically include high reliability, easy clearing of load circuit faults, and no high resonant current in the inverter. Disadvantages typically include complex controls, a large DC filtering inductor and limited control range of the inverter which requires pre-regulation of the DC voltage. Furthermore, if a phase controlled rectifier is used as a pre-regulator, the power factor over a lower part of the control range is not optimal.
It is also advantageous to use a plurality of induction furnaces connected to a common power source. U.S. Pat. No. 5,508,497 to Fabianowski et al. discloses such a system to decrease the maximum amount of power used at any one time. U.S. Pat. No. 5,272,719 to Cartlidge et al. discloses a system that uses a single power supply to feed an induction holding furnace and an induction melting furnace.
What is desired, therefore, is an induction furnace converter that reliably operates at the resonant frequency of the load circuit without susceptibility to major damage from load circuit faults and that operates with a satisfactory power factor at all utility power levels. Additionally, it desired to have a induction furnace system that uses a common power supply to supply power to a plurality of induction furnaces.